I considered the ISS speed v as 8000 m/s or 0.00002667c (c is the speed of light). Then I calculated the epsilon factor as epsilon = sqrt ( 1 – v^2 / c^2 ) = 0.9999999996443555 Finally I applied the epsilon factor to the ISS orbit time (3013 days * epsilon) and found out that the resulting difference is 0.0925 seconds.

That means that time inside the ISS has so far been about one tenth of a second slower than the time down here on earth.

Waitaminute, slower? That doesn’t grok with gravity slowing down time. A closer look at the above math reveals it is calculating the effects of velocity on time. As objects approach the speed of light, their relative time slows down. This fact fuels an interesting thought experiment known as the Twin Paradox, where one twin goes on a light speed journey and the other stays on Earth. For the traveling twin only a few moments pass during the journey, but for the twin on Earth, it has been decades.

ISS Science Officer Ed Lu independently confirms the ISS math, concluding the astronauts age 0.007 seconds less than people on Earth. So these two sources have figured out how the ISS’s velocity slows down its relative time, but there’s no mention of gravity in their math.

If the ISS were to orbit the Earth at 1.5 times the Earth’s radius (5,900 miles or 9500 km) then the effect of velocity and gravity on time would cancel each other out. At orbits greater than 5,900 miles, gravity is stronger and speeds up time, below 5,900 miles, velocity is stronger and time slows. The ISS orbits 255 miles above the Earth at 8000 m/s, so time runs approximately 0.0000000014 percent slower.

Ryan Somma has spent 15 years as a professional software developer currently working in Laboratory Information Management Systems (LIMS). This blog is a variety show of his various nerdy interests, from information science, to Enlightenment philosophy, to science fiction.